1. Field of the Invention
The invention relates generally to fiber optics and more particularly to a clamping device for positioning and retaining an optical fiber at the focal point of a lens.
2. DESCRIPTION OF THE PRIOR ART
In the field of optical communications, optical fibers are employed to carry data from a transmitting station to a receiving station. In order to transmit an optical signal from a light source via a first optical fiber to another optical conducting element, e.g., a second optical fiber, an optical lens is employed to redirect the light rays of the optical signal. Each optical fiber has an endface in which the optical signal must pass through. To prevent attenuation of the optical signal, the optical fiber endface must interface with the focal point along the optical axis of the lens.
In the past, several techniques have been utilized to align the optical fiber to the lens both in a forward and a lateral mode. Each of the techniques employed one of a plurality of lens contacts for terminating the optical fiber. A first of these techniques teaches that the lens contact is terminated by micropositioning a "pigtailed" optical fiber, that is, the fiber is back illuminated and optimized in position. The optimized position of the fiber is accomplished by projecting the back illumination of the fiber through the lens to a target on a screen. When the back illumination through the lens strikes the target on the screen, the fiber is bonded securely and thus optimized in position with the fiber endface interfacing the focal point of the lens. A second of these alignment techniques involves providing a cylindrical hole behind the lens. The cylindrical hole is aligned along the optical axis of the lens and is blind ended at the focal point of the lens. Also, the cylindrical hole has a diameter that accommodates the optical fiber with a close fit and the optical fiber is bonded within the cylindrical hole with an index of refraction matched adhesive.
The process of aligning the optical fiber to the lens requires achieving both forward alignment and lateral alignment of the optical fiber which is usually a problem. In the first technique the problem manifests itself in the tedious requirement of adjusting the optical fiber to a stringent tolerance to permit the back illumination through the lens to strike the target. A second problem that exists concerns the necessary use of an epoxy to affix the optical fiber to the lens. In the back illumination method, epoxy is required to securely bond the fiber to the lens. In the second technique of alignment utilizing the cylindrical hole in the lens, the epoxy produces a hydraulic effect when the fiber is inserted into the cylindrical hole. The epoxy which is an index of refraction matched adhesive creates a hydraulic force that prevents the fiber endface from reaching the focal point of the lens. This problem results in a loss of or attenuation of the optical signal.
Several prior art references which are relevant to the instant invention have been discovered and will be discussed briefly.
In Contina, British Pat. No. 1,017,354, there is disclosed a system in which light is transmitted from one element to another so that the difference in cross-sections between the elements can be effected within a very short distance without loss of intensity. An optical system comprised of two bundles of optical fibers is arranged with one end of a first bundle facing one end of a second bundle and spaced apart therefrom. The ends of the two bundles are of different areas and a transparent body optically couples the two ends together. The transparent body has a reflecting surface which is substantially ellipsoidal in shape and coaxial with the bundle ends. Light rays of the optical signal are reflected such that they pass through a focal point in an axial endface of the second bundle.
In Kahn, British Pat. No. 1,429,843, there is disclosed a coupling unit consisting of a first housing member and a second housing member each carrying an end region of one of two light guides. The first housing member has a first coupling formed to fit into a second coupling of the second housing member. The first light guide is held centrally in the first coupling by a first body and the second light guide is held centrally in the second coupling by a second body. During manufacture, the ends of the light guides may be polished with the faces of the first and second bodies. A lens is mounted between the ends of the two light guides. Accurate alignment between the lens and the light guide is required.
In Cook, et al., U.S. Pat. No. 3,950,075, there is disclosed an optical communication system which includes a diode for emitting optical wave energy to be coupled to an optical waveguide bundle. A transparent spherical bead is axially aligned with the bundle and the center of the light emitting area of the diode. The bead is securely mounted in an aperture which extends through one end of an insert within a housing. Precise alignment of the center of the bead along the axis of the aperture results from a force fit. The diode is disposed in the housing for proper alignment of the light emitting area of the diode with the bead. The end of the bundle is disposed in a termination ferrule having an aperture, the bundle being potted in the ferrule by a bonding material. The ferrule maintains the fiber bundle in a substantially parallel alignment. A problem is the high tolerance alignment required between the diode, the bead and the bundle.
In Hunzinger, U.S. Pat. No. 4,102,559, there is disclosed an optical structure comprising two very thick plano-convex lenses with a common axis. The convex surfaces of the lenses face each other while a radiation source and an input face of a fiber are each disposed on one of the convex surfaces in the vicinity of the axis. The main feature of the device resides in the considerable thickness of the lenses relative to the radii of curvature of the dioptric faces. As a result, aberrations, in particular spherical aberrations, are reduced while the dimensions of the device are such as to enable easy construction and handling.
In Chown, U.S. Pat. No. 4,147,402, there is disclosed a process for manufacturing a lens termination for an optical fiber using laser machining to form a cavity which centers the fiber in the termination. A plastic preform is provided comprising a tube member with a coaxial bore having one end closed by a lens at least the rearward portion of which comprises a plastic material. The bore and lens are arranged such that the base of the bore lies in the focal plane of the lens. Light is directed from a laser through the lens so as to focus the light onto the base of the bore thus evaporating a portion of the plastic material to form a cavity which, when the termination is in use, receives the bared end of a coated optical fiber. An index matching adhesive is used to bond the fiber within the cavity which produces the previously described hydraulic force preventing the fiber endface from reaching the focal plane of the lens.
In Rush et al., U.S. Pat. No. 4,183,618, there is disclosed an optical waveguide including a connector having a longitudinal bore in which an end of an optical waveguide is to be anchored, lens means for transmitting light from one end of the waveguide and defining a recess including a focal point of the lens means, and a housing in which the connector and lens means are mounted. The terminal also includes an elongate optical member having first and second ends with respective end faces, the first end of the elongate optical member extending partially through the connector and being anchored therein, and the second end of the elongate optical member being mounted in the recess in the lens means with its end face disposed at the focal point of the lens means. The connector includes means for maintaining the first end of the optical member and an end of an optical waveguide in alignment with one another.
In Knoe et al., U.S. Pat. No. 4,327,963, there is disclosed a coupling element for an optical transmission system having a coupling element comprised of a convex lens with a refractive index. The refractive index is dependent on a radius "r" of a shell in the lens. The coupling element further includes a holder through which a capillary extends. The convex lens which is spherical is affixed to an end of an optical fiber with the lens-fiber interface being enclosed within the capillary. The capillary is preferably circular cylindrical in shape and may rest in a v-shaped groove of a holder. A transparent coupling medium may be disposed between the lens and the fiber for light refraction purposes.
Further, the prior art discloses a hermaphroditic rugged optical fiber connector which uses fibers terminated with miniature lenses and intended for use under adverse field conditions. A microlens is positioned on the end of each fiber to form an expanded beam termination which reduces the effect of dirt on the connector performance and provides easy cleaning. Two expanded beam terminations are housed in the rear insert, with a cable strength member clamp, which has a high tensile performance, and are located in a precision front insert in the connector body. A cable end and a plurality of fiber terminations are protected from the environment by being sealed in the connector body cavity. A replaceable protective window forms the cavity seal at the front of the connector while an O-ring in the endbell nut seals the cable entry.
After consideration of the known prior art relevant to the instant invention, the problems involving tedious forward and lateral alignment of the optical fiber to the lens and the necessary use of epoxy to affix the optical fiber to the lens still exist.